Pushing the Thermal Conductivity Limit by Decoupling Dual-Channel Phonon Transport in Crystals

We propose a novel design principle for achieving ultralow thermal conductivity in crystalline materials via a "heavy-light and soft-stiff" structural motif. By combination of heavy and light atomic species with soft and stiff bonding networks, both particle-like (κ_<i>p</i>) and wave-like (κ_<i>c</i>) phonon transport channels are concurrently suppressed. First-principles calculations show that this architecture induces a hierarchical phonon spectrum: soft-bonded heavy atoms generate dense low-frequency modes that enhance scattering and reduce κ_<i>p</i>, while stiff-bonded light atoms produce sparse high-frequency optical branches that disrupt the coherence and lower κ_<i>c</i>. High-throughput screening identifies Tl₄SiS₄ (κ_<i>p</i> = 0.10, κ_<i>c</i> = 0.06 W/mK) and Tl₄GeS₄ (κ_<i>p</i> = 0.09, κ_<i>c</i> = 0.06 W/mK) as representative candidates with strongly suppressed transport in both channels. A minimal 1D triatomic chain model further demonstrates the generality of this mechanism, offering a new paradigm for phonon engineering beyond the conventional κ_<i>p</i>-κ_<i>c</i> trade-off.